Quite simply, this refers to a specific screen resolution — 3840 x 2160 — which is four times (hence the “4”) the amount of pixels you’d find on a non-4K display. 4K Ultra HD is the name ...

True 4k has a slightly different aspect ratio than most consumer displays, and runs at 4096 x 2160, whereas "ultra-HD" is the consumer term that technically refers to the aforementioned 3840 x 2160.